Creating a large landscape with accurately varied elevations
A map with 3D elevation can be generated from images.



In Blender 2.76, here is a workflow for generating elevation maps.


Obtain elevation data as an image. For example, potential sources are http://viewer.nationalmap.gov/basic/ , http://eros.usgs.gov/elevation-products , or http://earthexplorer.usgs.gov/ .
Import the image as a plane. This may require enabling the 'import as plane' add-on.





Go into edit mode and subdivide the plane lots of times - 10 to 16 times works well. The more times you subdivide, the finer the elevation will be, but the slower blender will respond.





Apply the displacement modifier to the plane. Choose the image that was imported as the texture.
Adjust the displacement modifier to get a good scale for the displacement.





For some visual effects, in Blender cycles, a color ramp can be used to color based on the altitude.






A blend file with a south-east use map imported is here: